Impressive advancements have been made in the field of gecko-inspired synthetic dry adhesives. A large range of manufacturing methods for these adhesives has been reported in the literature. However, the use of such adhesives in applications such as climbing has been much more limited, with just a few examples reported. It has generally been found that the adhesion levels generated in real-world climbing applications are significantly lower than those obtained using small samples in bench-top experiments.
One reason for this disparity is that, in addition to conforming to surfaces and generating useful levels of adhesion, the adhesives have additional requirements when used for climbing. The first of these requirements is controllability, i.e., the adhesives should not be sticky in the default state and adhere only when it is desirable. In any other case, energy will be wasted as it is expended in attaching and detaching the adhesive for each step. Controllability can be achieved by using switchable structures or by creating directional adhesive features whose adhesion generation is a function of applied shear load.
The second requirement is durability. The adhesives must undergo thousands of attach/detach cycles without significant loss of adhesive properties and, ideally, should resist fouling and be easy to clean. Durability is also correlated with controllability: gentle attach/detach cycles reduce mechanical wear and promote long life.
Micro-wedges are an example of a synthetic dry adhesive that has been successfully applied to climbing robots. Micro-wedges are simple, controllable, and durable structures that have enabled robots weighing 1 kg and more to climb on glass, plastic, wood paneling, painted metal, and similar surfaces. When unloaded, as shown in an oblique perspective view in FIG. 1a, they present a very small real area of contact with a surface and generate negligible adhesion. However, when loaded in a preferred shear direction, as shown in FIG. 1b, they bend, creating a larger contact area and generating adhesion that is proportional to the shear load. The micro-wedges' asymmetric taper ensures that the radius of curvature of the feature at the proximal edge of the contact patch increases with increasing shear load, allowing the tapered features to outperform features of constant cross-section at high shear loads. Furthermore, they may be easily cleaned using a piece of sticky tape.
In previous work, micro-wedges were manufactured by casting a polydimethylsiloxane (PDMS) silicone elastomer into molds created through a photolithographic process in which SU-8® photoresist (MicroChem Corp.) was subjected to two exposures, one angled, one vertical, through contact masks. The necessity of a thick photo-resist layer combined with the requirement for high precision alignment of exposures resulted in a time consuming, expensive mold fabrication process with relatively low yield.
What is needed is a method of fabricating controllable and durable synthetic dry adhesives that does not use an expensive photolithographic process, and which provides synthetic dry adhesives with performance in climbing and other applications that is comparable to or better than the micro-wedges used in previous work.